Triplicated digital systems are sometimes used in, for instance, telecommunication systems for safety reasons. In this respect, there are used three parallel branches in which identical operations are carried out. Downstream of given function blocks incorporated in said branches, a majority selection is made between mutually corresponding output signals from the three function blocks. Consequently, the malfunction of one of said three function blocks will have no significance. The system is thus tolerant to single errors and to multiple errors which do not overlap one another in time and space.
In order for a faulty or erroneous system function to be discovered, it is necessary to use the function concerned. System functions which remain unused over long periods of time can be encumbered with one or several latent errors which are not discovered and reported. Majority selection functions are additionally prone to this drawback, since this function is not tested until an error has occurred. In normal cases, i.e. when the function blocks are error-free, the output signals from the function blocks which correspond to one another in the three branches are equal. Consequently, latent errors may be present in the devices which carry out majority selection, these errors remaining undiscovered for as long as the function blocks are error-free and produce mutually identical output signals.
The devices which carry out majority selection comprise, for instance, logic networks composed of separate logic devices, e.g. NAND and/or NOR-gates, which contain, inter alia, two or more parallel-connected and series-connected transistors. A fault in such a transistor will normally remain undiscovered until a particular type of error occurs in a separate function block, which may not occur for a very long time, wherewith overlapping errors resulting in system malfunction may occur.